1. Mon. Not. R. Astron. Soc. 000, 000–000 (2002) Printed 1 February 2008 (MN L TEX style file v1.4)
A
The intermediate-age open cluster NGC 2158⋆
Giovanni Carraro1, L´o Girardi1,2 and Paola Marigo1†
e
1 Dipartimento di Astronomia, Universit` di Padova, Vicolo dell’Osservatorio 2, I-35122 Padova, Italy
a
2 Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, I-34131 Trieste, Italy
Submitted: October 2001
arXiv:astro-ph/0202018v1 1 Feb 2002
ABSTRACT
We report on U BV RI CCD photometry of two overlapping fields in the region
of the intermediate-age open cluster NGC 2158 down to V = 21. By analyzing
Colour-Colour (CC) and Colour-Magnitude Diagrams (CMD) we infer a reddening
EB−V = 0.55 ± 0.10, a distance of 3600 ± 400 pc, and an age of about 2 Gyr.
Synthetic CMDs performed with these parameters (but fixing EB−V = 0.60 and
[Fe/H] = −0.60), and including binaries, field contamination, and photometric errors,
allow a good description of the observed CMD. The elongated shape of the clump of
red giants in the CMD is interpreted as resulting from a differential reddening of about
∆EB−V = 0.06 across the cluster, in the direction perpendicular to the Galactic plane.
NGC 2158 turns out to be an intermediate-age open cluster with an anomalously low
metal content. The combination of these parameters together with the analysis of the
cluster orbit, suggests that the cluster belongs to the old thin disk population.
Key words: Open clusters and associations: general – open clusters and associations:
individual: NGC 2158 - Hertzsprung-Russell (HR) diagram
1 INTRODUCTION the present paper. Moreover this paper is the third of a
series dedicated at improving the photometry of northern
NGC 2158 (OCL 468, Lund 206, Melotte 40) is a rich
intermediate-age open clusters at Asiago Observatory. We
northern open cluster of intermediate age, located low in
already reported elsewhere on NGC 1245 (Carraro & Patat
the galactic plane toward the anti-center direction (α =
1994) and on NGC 7762 (Patat & Carraro 1995).
06h 07m .5, δ = +24◦ 06′ , ℓ = 186◦ .64, b = +1◦ .80, J2000.),
The plan of the paper is as follows. In Sect. 2 we summa-
close to M 35. It is classified as a II3r open cluster by Trum-
rize the previous studies on NGC 2158, while Sect. 3 is ded-
pler (1930), and has a diameter of about 5′ , according to
icated to present the observation and reduction strategies.
Lyng˚ (1987). It is quite an interesting object due to its
a
The analysis of the CMD is performed in Sect. 4, whereas
shape, for which in the past it was considered a possible glob-
Sect. 5 deals with the determination of cluster reddening,
ular cluster, also presenting an unusual combination of age
distance and age. Sect. 6 illustrates NGC 2158 kinematics.
and metallicity. In fact it is an intermediate-age open cluster,
Finally, Sect. 7 summarizes our findings.
but rather metal poor. It is a crucial object in determining
the Galactic disk abundance gradient and the abundance
spread at time and place in the disk.
The cluster is rather populous, and therefore it is an
ideal candidate to be compared with theoretical models of 2 PREVIOUS INVESTIGATIONS
intermediate-low mass stars (Carraro & Chiosi 1994a, Car- NGC 2158 has been studied several times in the past. The
raro et al. 1999). Since in the past no detailed studies have first investigation was carried out by Arp & Cuffey (1962),
been pursued with this aim, we decided to undertake a who obtained photographic BV photometry for about 900
multicolor CCD study of the cluster, which is presented in stars down to V = 18.5. Photographic photometry was also
obtained by Karchenko et al (1997) for more than 2000 stars
down to the same limiting magnitude together with proper
⋆ Based on observations carried out at Mt Ekar, Asiago, motions.
Italy. All the photometry is available at WEBDA database: CCD photometry in BV passbands was provided by
http://obswww.unige.ch/webda/navigation.html Christian et al. (1985) and Piersimoni et al. (1993). Both
† email: giovanni.carraro@unipd.it (GC); lgirardi@pd.astro.it these studies reach deeper magnitude limits. Anyhow, the
(LG); marigo@pd.astro.it (PM) former study basically provides only a selection of MS un-
c 2002 RAS

2. 2 Carraro et al.
Figure 1. A V map of the observed field from the photometry of one of the deep V frames; North is up and East is to the left; the
field is 9 × 11 arcmin2 . The circle confines the stars within 3′ from the cluster center. The size of each star is inversely proportional to
its magnitude.
evolved stars, whereas the latter one presents a nice CMD, Table 1. Journal of observations of NGC 2158 (January 6-7 ,
but the analysis of the data appears very preliminary. 2000).
There is some disagreement in the literature about the
value of NGC 2158 fundamental parameters, specially with Field Filter Time integration Seeing
respect to the cluster age. Estimates of cluster metallicities (sec) (′′)
have been obtained by several authors, and, although differ-
#1
ent, they all point to a sub-solar metal content ([Fe/H] =
U 240 1.2
−0.60, Geisler 1987, Lyng˚ 1987). Finally, the kinematics of
a
B 300 1.3
NGC 2158 has been studied by measuring spectra of giant V 120 1.3
stars (Scott et al. 1995; Minniti 1995) to provide radial ve- R 60 1.5
locities. It turns out that the mean cluster radial velocity is I 120 1.3
in the range 15 − 30 km/s (Scott et al. 1995). #2
U 240 1.2
B 300 1.1
V 120 1.3
3 OBSERVATIONS AND DATA REDUCTION R 60 1.5
I 120 1.3
Observations were carried out with the AFOSC camera at
the 1.82 m telescope of Cima Ekar, in the nights of January
6 and 7, 2000. AFOSC samples a 8′ .14 × 8′ .14 field in a
1K × 1K thinned CCD. The typical seeing was between 1.0 in U , 300 s in B, and 60–120 s in V RI. Several images
and 1.5 arcsec. were taken, either centered on the cluster core, or shifted
For NGC 2158, typical exposure times were of 240 s by about 4′ in order to better sample the neighboring field
c 2002 RAS, MNRAS 000, 000–000

3. The open cluster NGC 2158 3
Figure 2. Differences between standard magnitudes and those
obtained from our Eq. 1, for our standard stars and as a function
of colour.
Figure 3. Photometric errors as a function of magnitude, for our
NGC 2158 observations.
(see Fig. 1). However, only the images with the best seeing
were used. We also observed a set of standard stars in M 67
(Schild 1983; and Porter, unpublished). bration equations as a function of colour for all our standard
stars.
The data has been reduced by using the IRAF‡ pack-
Finally, Fig. 3 presents the run of photometric errors
ages CCDRED, DAOPHOT, and PHOTCAL. The calibra-
as a function of magnitude. These errors take into account
tion equations obtained (see Fig. 2) are:
fitting errors from DAOPHOT and calibration errors, and
u = U + 4.080 ± 0.005 + (0.010 ± 0.015)(U −B) + 0.55 X have been computed following Patat & Carraro (2001). It
b = B + 1.645 ± 0.010 + (0.039 ± 0.015)(B −V ) + 0.30 X can be noticed that stars brighter than about 20 in V , R,
and I, 21 in B, and U , have photometric errors lower than
v = V + 1.067 ± 0.011 − (0.056 ± 0.018)(B −V ) + 0.18 X 0.1 mag. The final photometric data is available in electronic
r = R + 1.109 ± 0.012 − (0.075 ± 0.032)(V −R) + 0.13 X form at the WEBDA§ site.
i = I + 1.989 ± 0.048 + (0.118 ± 0.145)(R−I) + 0.08 X
(1)
where U BV RI are standard magnitudes, ubvri are the in- 4 THE COLOUR-MAGNITUDE DIAGRAMS
strumental ones, and X is the airmass. For the extinction A comparison of our photometry with past analyses is shown
coefficients, we assumed the typical values for the Asiago in Fig. 4, from which it is evident that the present study su-
Observatory. Figure 2 shows the residuals of the above cali- persedes the previous ones. In fact, we reach V = 21, and are
able to cover all the relevant regions of the CMD. Instead,
Arp & Cuffey (1958) photometry extends only for a cou-
‡ IRAF is distributed by the National Optical Astronomy Obser- ple of magnitudes below the turn-off point (TO) , whereas
vatories, which are operated by the Association of Universities for
Research in Astronomy, Inc., under cooperative agreement with
the National Science Foundation. § http://obswww.unige.ch/webda/navigation.html
c 2002 RAS, MNRAS 000, 000–000

4. 4 Carraro et al.
Figure 4. BV CMDs of NGC 2158. The left panel presents the Arp & Cuffey (1962) photometry, the central panel the Christian et al.
(1985) photometry, whereas the right panel shows our photometry.
the photometry of Christian et al. (1985) does not cover the 5 CLUSTER FUNDAMENTAL PARAMETERS
evolved region of the CMD.
There fundamental parameters of NGC 2158 are still con-
troversial in the literature (see Table 2). The cluster age
To better identify the TO location and the Red Giant estimates range from 0.8 to 3.0 Gyr, the distance from 3500
(RG) clump, in Fig. 5 we plot the CMDs obtained by con- to 4700 pc and the reddening EB−V from 0.35 to 0.55. In
sidering stars located in different cluster regions. In details, the next sections we are going to derive update estimates
left panel presents the CMD obtained by including all the for NGC 2158 basic parameters.
measured stars, central panel considers the stars within a
circle of radius 3′ , whereas the right panel presents only
the stars located inside a circle of radius 1.5′ . The radius 5.1 Reddening
adopted in the central panel is compatible with the avail- In order to obtain an estimate of the cluster mean reddening,
able estimate of the cluster diameter, which is about 5′ , so we analyse the distribution of the stars with V < 17 in the
that we are likely considering most of the cluster members. (B − I) vs. (B − V ) plane, which is shown in Fig. 6.
By inspecting this CMD, we find that the TO is located at The linear fit to the main sequence in the (B − I) vs.
V ≈ 16.0, (B − V ) ≈ 1.0, whereas a prominent clump of (B − V ) plane,
He burning stars is visible at V ≈ 15.0, (B − V ) ≈ 1.5.
The diagonal structure of this latter is probably due to dif- (B − I) = Q + 2.25 × (B − V ) (2)
ferential reddening effects, which we are going to discuss in can be expressed in terms of EB−V , for the RV = 3.1 ex-
Sect. 5.3. The MS extends for 5 magnitudes, getting wider tinction law, as
at increasing magnitudes: this is compatible with the trend
of photometric errors (see Fig. 3) and the probable presence Q − 0.014
EB−V = , (3)
of a significant population of binary stars. The global CMD 0.159
morphology resembles that of NGC 7789 (Vallenari et al. following the method proposed by Munari & Carraro
2000) and NGC 2141 (Carraro et al. 2001), two well studied (1996a,b). This method provides a rough estimate of the
rich intermediate-age open clusters. mean reddening and, as amply discussed in Munari & Car-
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5. The open cluster NGC 2158 5
Figure 5. BV CMDs of NGC 2158. The left panel presents the CMD obtained by including all the measured stars, the central panel
considers the stars within a radius of 3′ , whereas the right panel shows only the stars located inside a radius of 1.5′ .
Table 2. NGC 2158 fundamental parameters taken from the literature.
Arp & Cuffey Christian et al. Kharcenko et al. Piersimoni et al.
E(B − V ) 0.43 0.55 0.35 0.55
(m − M ) 14.74 14.40 13.90 15.10
distance (pc) 4700 3500 3700 4700
Age (Gyr) 0.8 1.5 3.0 1.2
raro (1996a), can be used only for certain colour ranges. fect. The solid line is an empirical Zero-Age Main Sequence
In particular Eq. (3) holds over the range −0.23 ≤ (B − (ZAMS) taken from Schmidt-Kaler (1982), whereas the
V )0 ≤ +1.30. MS stars have been selected by considering dashed line is the same ZAMS, but shifted by EB−V = 0.55.
all the stars within 3′ from the cluster center and having The ratio EU −B /EB−V = 0.72 has been adopted. This shift,
17 ≤ V ≤ 21 and 0.75 ≤ (B − V ) ≤ 1.25. A least-squares together with the dispersion of the data around the shifted
fit through all these stars gives Q = 0.097, which, inserted ZAMS, provides the reddening value of EB−V = 0.55 ± 0.10.
in Eq. (3), provides EB−V = 0.56 ± 0.17. The uncertainty
is rather large, and is due to the scatter of the stars in this
plane, which indicates the presence of stars with different 5.2 Distance and age
reddening, presumably a mixture of stars belonging to the
cluster and to the field. As already mentioned, there is still a considerable disper-
sion in the literature among different estimates of NGC 2158
Another indication of the cluster mean reddening can be distance and age. We have derived new estimates for these
derived from the Colour-Colour diagram (U −B) vs. (B−V ), parameters as follows.
shown in Fig. 7. Here we consider again all the stars located First, from the Girardi et al. (2000a) database we gen-
within 3′ from the cluster center having 17 ≤ V ≤ 21 and erate theoretical isochrones of metallicity Z = 0.0048, that
0.75 ≤ (B − V ) ≤ 1.25, to alleviate the contamination ef- corresponds to the observed value of [Fe/H] = −0.60. Fig-
c 2002 RAS, MNRAS 000, 000–000

6. 6 Carraro et al.
Figure 6. NGC 2158 MS stars within 3′ in the (B −V ) vs. (B −I) Figure 7. NGC 2158 stars within 3′ in the colour-colour diagram.
plane. The solid line is an empirical ZAMS taken from Schmidt-Kaler
(1982), whereas the dashed line is the same ZAMS, but shifted
by EB−V = 0.55. The arrow indicates the reddening law.
ure 8 shows the isochrones with ages between 1.58 to 2.51
Gyr, which defines the age interval compatible with the
observed magnitude difference between the red clump and
the turn-off region. The isochrones were shifted in apparent
magnitude and colour, until the locus of core-helium burn-
ing stars coincided with the observed mean position of the
clump. The results, shown in Fig. 8, imply a true distance
modulus of (m−M )0 = 12.8 mag (3630 pc), and a colour ex-
cess of EB−V = 0.60 for NGC 2158. This value is compatible
with the one obtained in the previous Sect. 5.1.
It should be remarked that these are just first estimates
of the cluster parameters, that we will now try to test further
by means of synthetic CMDs. Figure 9 shows the sequence
of steps required to simulate a CMD aimed to reproduce the
NGC 2158 data. These steps are:
• The 2-Gyr old isochrone of Z = 0.0048 is used to simu-
late a cluster with 100 red clump stars. Assuming a Kroupa
(2001) IMF, in order to reach this number we need an ini-
tial cluster mass of about 1.5 × 104 M⊙ , which is assumed
herein-after. We have simulated detached binaries, assuming
that 30 percent of the observed objects are binaries with a
mass ratio located between 0.7 and 1.0. This prescription is
in agreement with several estimates for galactic open clus-
ters (Carraro & Chiosi 19984ab) and with the observational Figure 8. NGC 2158 data the V vs. B − V diagram (points), as
data for NGC 1818 and NGC 1866 in the LMC (Elson et compared to Girardi et al. (2000a) isochrones of ages 1.58 × 109 ,
al. 1998; Barmina et al. 2001). The result of such simulation 2.00 × 109 , and 2.51 × 109 yr (solid lines), for the metallicity
is shown in Fig. 9(a). In this panel we can see that most of Z = 0.0048. A distance modulus of (m − M )0 = 12.8, and a
the stars – the single ones – distribute along the very thin colour excess of EB−V = 0.60, have been adopted. The direction
sequence defined by the theoretical isochrone. Binaries ap- of the reddening vector is indicated by the arrow at the bottom
left.
pear as both (i) a sequence of objects roughly parallel to the
main sequence of single stars, and (ii) some more scattered
objects in the evolved part of the CMD.
• In order to estimate the location of foreground and
background stars, we use a simple Galaxy model code (Gi-
rardi et al., in preparation). It includes the several Galac-
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7. The open cluster NGC 2158 7
Figure 9. Simulations of NGC 2158 and its field in the V vs. B − V diagram. (a) Simulation of a 2-Gyr old cluster with Z = 0.0048
and a initial mass of 1.5 × 104 M⊙ , based on the same isochrones, distance modulus and colour excess as in Fig. 8. We have assumed
that 30 percent of the stars are binaries with mass ratios between 0.7 and 1.0. (b) Simulation of a 9 × 11arcmin2 field centered at
galactic coordinates ℓ = 186◦ .64, b = +1◦ .80, performed with Girardi et al. (2001) Galactic model. Panels (c) and (d) are the same as
(a) and (b), respectively, after simulation of photometric errors. Panel (e) shows the sum of (c) and (d), that can be compared to the
observational data shown in panel (f ).
tic components – thin and thick disk, halo, and an extinc- with z0 = 95 pc, t0 = 4.4 Gyr, α = 1.66. The simulated field
tion layer – adopting geometric parameters as calibrated by has the same area (9 × 11 arcmin2 ) and galactic coordinates
Groenewegen et al. (2001). The most relevant component in (ℓ = 186◦ .64, b = +1◦ .80) as the observed one for NGC 2158.
this case is the thin disk, which is modelled by exponential The results are shown in Fig. 9(b). It is noteworthy that, in
density distributions in both vertical and radial directions. this direction, most of the Galactic field stars appear in a
The radial scale heigth is kept fixed (2.8 kpc), whereas the sort of diagonal sequence in the CMD, that roughly corre-
vertical scale heigth hz increases with the stellar age t as sponds to the position of NGC 2158 main sequence.
hz = z0 (1 + t/t0 )α (4) • We then simulate the photometric errors as a function
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8. 8 Carraro et al.
Figure 10. The same simulations of Fig. 9, but detailing the region of clump giants. (a) The simulated data. Circles are single stars
in the cluster, squares are the binaries, and the few triangles are field stars. (b) The same as in panel (a), but including the simulation
of photometric errors. (c) The observed data for NGC 2158. The arrow shows the reddening vector corresponding to ∆EB−V = 0.1.
of V magnitude, with typical values derived from our ob- 1.5 × 104 M⊙ , well represent the cluster parameters. All
servations (see Fig. 3). The results are shown separately for these values are uncertain to some extent:
cluster and field stars in panels (c) and (d) of Fig. 9.
• From Fig. 8, we can estimate a maximum error of 15
• The sum of field and cluster simulations is shown in
percent (0.3 Gyr) in the age. One should keep in mind, how-
Fig. 9(e). This can be compared directly to the observed
ever, that the absolute age value we derived, of 2 Gyr, de-
data shown in Fig. 9(f).
pends on the choice of evolutionary models, and specially on
the prescription for the extent of convective cores. For the
The comparison of these two latter panels indicates that stellar masses envolved (MTO ∼ 1.5 M⊙ for NGC 2158), our
the selected cluster parameters – age, metallicity, mass, dis- models (Girardi et al. 2000a) include a moderate amount of
tance, reddening, and binary fraction – really lead to an ex- core overshooting.
cellent description of the observed CMD, when coupled with • Our best fit model corresponds to EB−V = 0.60, which
the simulated Galactic field. The most noteworthy aspects is compatible with the range EB−V = 0.55 ± 0.1 indicated
in this comparison are the location and shape of the turn-off in Sect. 5.1. The uncertainty of 0.1 mag in EB−V causes
and subgiant branch, that are the features most sensitive to an uncertainty of ∼ 0.3 mag in the distance modulus (15
the cluster age. percent in distance).
• The metallicity cannot be better constrained from the
Of course, there are minor discrepancies between the ob-
CMD data, unless we have more accurate estimates of the
served and simulated data, namely: (i) The simulated cluster
reddening.
is better delineated in the CMD than the data. This may
• The initial mass estimate depends heavily on the choice
be ascribed to a possible underestimate of the photometric
of IMF, that determines the mass fraction locked into low-
errors in our simulations, and to the possible presence of
mass (unobserved) objects. The value of 1.5 × 104 M⊙ was
differencial reddening across the cluster (see next section).
obtained with a Kroupa (2001) IMF, corrected in the lowest
(ii) There is a deficit of simulated field stars, that can be
mass interval according to Chabrier (2001; details are given
noticed more clearly for V < 16 and (B − V ) < 1. This
in Groenewegen et al. 2001), and should be considered just
is caused by the simplified way in which the thin disk is
as a first guess. At present ages, supernovae explosions and
included in the Galactic model: it is represented by means
stellar mass loss would have reduced this mass by about 20
of simple exponentially-decreasing stellar densities in both
percent.
radial and vertical directions, and does not include features
such as spiral arms, intervening clusters, etc., that are nec-
essary to correctly describe fields at low galactic latitudes.
Anyway, the foreground/background simulation we present 5.3 Red clump structure and differential
is only meant to give us an idea of the expected location of reddening
field stars in the CMD. Our cluster simulations also allows us to examine in better
Although these shortcomings in our simulations might detail the observed structure of the red clump in NGC 2158.
probably be eliminated with the use of slightly different pre- Figure 10 details the clump region in the CMD, for both
scriptions, they do not affect our main results, that regard simulations (panels a and b) and data (panel c). As can
the choice of cluster parameters. be readily noticed in panel (c), the observed clump appears
Then, we conclude that (m−M )0 = 12.8 mag (3630 pc), as a diagonal structure, whose slope is roughly coincident
EB−V = 0.60, 2 Gyr, Z = 0.0048 ([Fe/H] = −0.60), and with the reddening vector. In cluster simulations, however,
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9. The open cluster NGC 2158 9
Figure 11. Bottom panels: The same as in Fig. 10, but now illustrating the stellar data at different intervals of galactic latitude, namely
(a) 1◦ .73 < b < 1◦ .77 (toward NE in Fig. 1), (b) 1◦ .77 < b < 1◦ .81 (central part of the cluster), and (c) 1◦ .81 < b < 1◦ .85 (toward
SW). In all cases, only stars located at 186◦ .58 < ℓ < 186◦ .69 (a strip about 7′ wide) were plotted. As a reference to the eye, we plot
also the same 2-Gyr old isochrone as shown in previous Fig. 8. The panels at the top illustrate, for each case, the colour histogram of
stars with 14.7 < V < 15.7. Notice the progressive shift of the clump to the blue as b increases.
the clump is normally seen as a more compact structure. isochrone, located at a fixed position in all panels, allows an
This can be appreciated in the simulations for single stars easy visualization of how the clump gets bluer at increas-
shown in Girardi et al. (2000b), where the model clumps are ing b.¶ . Assuming that this effect is caused by differential
found to be more elongated in the top-bottom direction in reddening perpendicularly to the Galactic plane, we get an
the CMD, and not diagonally. The same result is found in estimate of ∆EB−V /∆b ≃ −0.011 mag/arcmin.
Figure 10a, if we look only at the locus of single stars. We conclude that NGC 2158 presents some amount of
However, part of the clump widening might be caused differential reddening. Along the ∼ 6′ diameter of the clus-
by the presence of binaries, as shown by the different sym- ter, this effect amounts to about ∆EB−V ∼ 0.06 mag. Since
bols in our simulations of panels (a) and (b). It turns out our previous determinations of the cluster age, distance, and
that binaries composed of clump plus turn-off stars are lo- reddening were based on the mean location of the observed
cated at a bluer colour, and are slightly brighter, if com- stars, the correction of the data for differential reddening
pared to the locus of single clump stars. Thus, binaries tend would not imply any significant change in the derived pa-
to widen the clump structure along a direction that roughly rameters.
coincides with the reddening vector. But anyway, as can be
readily noticed from Fig. 10, binaries cannot account entirely
for the clump elongation observed in NGC 2158.
Instead, since NGC 2158 is located very low in the 6 CLUSTER KINEMATICS
Galactic plane, the observed clump morphology might well The availability of mean radial velocity and proper motion
be caused by the presence of differential reddening over the measurements allows us to discuss in some detail the kine-
cluster. We can get an estimate of the expected differential matics of NGC 2158. Radial velocity has been measured for
reddening, starting from simple models of the dust distri- 8 stars by Scott et al. (1995) and for 20 stars by Minniti
bution in the thin disk. To this aim, we use the Girardi et (1995). These measurements have a comparable accuracy
al. (2001) Galactic model, assume a local extinction value between 10 and 15 km/s. A systematic shift of about 10
of 0.75 mag/kpc in V (Lyng˚ 1982), and a diffuse dust layer
a km/s is likely to exist, in the sense that the mean radial
of exponentially-decreasing density with a scale height equal velocity from Minniti (1995) is lower than that derived by
to 110 pc. With these parameters, in correspondence of the Scott et al. (1995). Although Minniti (1995) mean radial ve-
NGC 2158 location we obtain a differential reddening of locity is based on better statistics, we shall present results
∆EB−V /∆b = −0.021 mag/arcmin perpendicularly to the based upon both the determinations.
Galactic plane. This estimate has the right order of magni- Absolute proper motions have been derived by
tude to explain the width of the observed clump.
In order to investigate whether this kind of picture is
realistic, in Fig. 11 we plot the CMDs for NGC 2158, sepa- ¶ A similar effect was also noticed for NGC 2158 main sequence
rated in different strips of Galactic latitude b. The 2-Gyr old stars.
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10. 10 Carraro et al.
Kharchenko et al. (1997), and amount to µx = +0.66 ± 2.03,
µy = −3.23 ± 2.16, where µx = µα · cosδ and µy = µδ . 0.3
NGC 2518 Vr = 28 Km/s
Following in details Carraro & Chiosi (1994b) and Bar- 0.2
bieri & Gratton (2001) we derived the velocity components 0.1
z (kpc)
of NGC 2158 in a Galactocentric reference frame U , V and
0
W . The results are summarized in Table 3.
By adopting the Allen & Santill´n (1991) rotationally
a -0.1
symmetric Galaxy mass model, we integrated back in time -0.2
NGC 2158 orbit for a duration comparable with NGC 2158 -0.3
age (see previous Sect. 5.2), in order to obtain estimates 8 9 10 11 12
of its eccentricity, epiciclycal (ω-) and vertical (z-) ampli-
tude. These parameters, together with age and metallicity, 0.3
NGC 2518 Vr = 14 Km/s
are fundamental to place the cluster in the right disk popu- 0.2
lation.
0.1
z (kpc)
The orbit integration has been performed using a mod-
ified version of the second-order Burlish-Stoer integrator 0
originally developed by S.J. Aarseth (private communica- -0.1
tion). We provide orbits both for the Minniti (1995) and -0.2
Scott et al. (1995) mean radial velocity estimates. They are -0.3
shown in the upper and middle panels of Fig. 12. The pa- 8 9 10 11 12
rameters are basically consistent, as listed also in Table 4.
For the sake of the discussion, in the lower panel we show 2
a new orbit determination for the open cluster NGC 2420, 1.5 NGC 2420
which roughly shares the same age (1.8 Gyr) and metallicity 1
z (kpc)
([Fe/H] = −0.42) of NGC 2158 (Friel & Janes 1993; Carraro 0.5
0
et al 1998). The orbit of NGC 2420 was previously com- -0.5
puted by Keenan & Innanen (1974) who suggest that this -1
cluster might have been disturbed in his motion around the -1.5
Galactic center by the influence of the Magellanic Clouds, -2
an hypothesis which sounds reasonable - the cluster has high 8 9 10 11 12 13 14
eccentricity, large apogalacticon and stays most of the time ϖ (kpc)
relatively high above the galactic plane- but which deserves
Figure 12. NGC 2158 orbit in the meridional plane. In the upper
a further detailed numerical investigation.
panel we display the orbit obtained by using the radial velocity
Christian et al. (1985) argue about the possibility that estimate from Scott et al. (1995), whereas the middle panel shows
NGC 2158 and NGC 2420 might share common properties the orbit obtained by adopting the radial velocity estimate from
and origin, since they are coeval and have very low metal Minniti (1995). The lower panel presents the orbit of NGC 2420.
abundances for open clusters of this age. It is therefore in-
teresting to compare their orbits, also because NGC 2158
is even metal poorer than NGC 2420. With an eccentricity Table 4. NGC 2158 orbit’s basic parameters.
e = (Ra − Rp )/(Ra + Rp ) = 0.20 – where Ra and Rp are the
apo- and peri-galacticon, respectively – the cluster reaches a Ra Rp e zmax
maximun distance of about 12 kpc from the Galactic Center kpc kpc kpc
in the direction of the anti-center, where it is located right
now and where it probably formed. It remains relatively low Minniti 12.21 8.11 0.20 0.21
in the Galactic disk, in a region populated by young and Scott et al. 12.19 8.03 0.21 0.20
intermediate-age Population I objects. The only difference
with this population is the rather low metal content, less
than half the solar value. formed from the same material. Typically, the mean metal
Apparently, we are facing two significantly different or- content of the Galactic disk at distances between 12 and 16
bits. NGC 2158 has an orbit more similar to normal Popula- kpc ranges between [Fe/H] = −0.50 and [Fe/H] = −0.70, ac-
tion I objects, whereas NGC 2420 possesses an eccentricity cording to recent estimates of the Galactic disk metallicity
much higher than the typical Population I objects. More- gradient (Carraro et al 1998). Such a low metal content is
over, NGC 2420 is more distant than NGC 2158. NGC 2420 compatible with the low density, hence low star formation,
and NGC 2158 are not the only two cases of low metal- probably typical of that region. In this respect it would be
licity intermediate-age clusters in the anti-center direction. interesting to compute Galactic orbits for all these clusters
From Carraro et al. (1998) we have extracted 10 clusters to check whether a trend exists to have more eccentric or-
with age between 1.5 and 3.5 Gyr and low metal content bits at increasing Galactocentric distance in this region of
([Fe/H] < −0.50). All these clusters presently lie in a Galac- the anti-center. This would help to better understand the
tic sector between ℓ = 135◦ and ℓ = 225◦ . They are: structure and evolution of the outer Galactic disk.
NGC 2158, NGC 2204, NGC 2420, NGC 2141, NGC 2243,
Tombaugh 2, Berkeley 19, 20, 21, 31 and 32. The common
properties of these clusters suggest the possibility that they
c 2002 RAS, MNRAS 000, 000–000

11. The open cluster NGC 2158 11
Table 3. NGC 2158 basic kinematical parameters. The velocity components have been computed by adopting Minniti (1995, first row)
Scott et al (1995, second row) radial velocity estimates.
(m − M )0 X Y Z U V W
kpc kpc kpc km/s km/s km/s
12.80 12.11 -0.42 0.11 -7.99 -56.50 -16.88
-21.88 -58.13 -16.45
7 CONCLUSIONS Groenewegen M.A.T., Girardi L., Hadziminaoglou E., et al. 2001,
A&A, submitted
We have presented a new CCD U BV RI photometric study Lyng˚ G., 1982, A&A 109, 213
a
of the intermediate age open cluster NGC 2158. From the Lyng˚ G., 1987, The Open Star Clusters Catalogue, 5th edition
a
analysis of the available data we can draw the following con- Keenan D.W., Innanen K.A., 1974, ApJ 189, 205
clusions: Kharcenko N., Andruk V., Schilbach E., 1997, Astron. Nach. 318,
253
• The age of NGC 2158 is about 2 Gyr, with a 15 % un- Kroupa P., 2001, MNRAS 322, 231
certainty; Minniti D., 1995, A&AS 113, 299
• the reddening EB−V turns out to be 0.55 ± 0.10 and we Munari U., Carraro G., 1996a, A&A 314, 108
find evidence of differential reddening (of about 0.06 mag) Munari U., Carraro G., 1996b, MNRAS 283, 905
across the cluster; Patat F., Carraro G., 1995, A&AS 114, 281
• we place the cluster at about 3.6 kpc from the Sun Patat F., Carraro G., 2001, MNRAS 325, 1591
toward the anti-center direction; Piersimoni A., Cassisi S., Brocato E., Straniero O., 1993,
Mem.S.A.It. 64, 609
• combining together NGC 2158 age, metallicity and kine-
Scott J.E., Friel E.D., Janes, K.A., 1995, AJ 109, 1706
matics, we suggest that it is a genuine member of the old Schild R.E., 1983, PASP 95, 1021
thin disk population. Schmidt-Kaler, Th., 1982, Landolt-B¨rnstein, Numerical data
o
and Functional Relationships in Science and Technology, New
Series, Group VI, Vol. 2(b), K. Schaifers and H.H. Voigt Eds.,
Springer Verlag, Berlin, p.14
ACKNOWLEDGEMENTS Trumpler R.J., 1930, Lick Observ. Bull. 14, 154
We are very grateful to Chiara Miotto for carefully reading Vallenari A., Carraro G., Richichi A., 2000, A&A 353, 147
this manuscript, to Mauro Barbieri for NGC 2158 orbit inte-
gration, to Martin Groenewegen for the latest calibration of
the Galactic model, and to Luciano Traverso, who secured
the observations of January 7. We acknowledge also the ref-
eree, dr. G. Gilmore, for his useful suggestions. This study
has been financed by the Italian Ministry of University, Sci-
entific Research and Technology (MURST) and the Italian
Space Agency (ASI), and made use of Simbad and WEBDA
databases.
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